The packing ordinarily used comprises corrugated strips comprising alternate parallel corrugations each disposed in a general vertical plane and against each other, the undulations being oblique and descending in opposite directions from one strip to the next. The degree of perforation is about 10% for this so-called cross corrugated packing.
GB 1 004 046 discloses packings of the cross corrugation type.
CA-1 095 827 provides an improvement of this type of packing by adding dense perforations of small diameter to permit the liquid to move from one side to the other of the cross corrugated strips.
WO 94/12258 provides an improvement of this type of packing based on exact positioning of the strips relative to each other in a vertical plane, by a system of interlocking. This device has for its object to provide more packing surface in a same volume, because the interlocking permits an interpenetration of the strips.
WO 86/06296 and WO 90/10497 disclose a packing comprising horizontal superposed layers, each layer comprising rows of pyramids.
In WO 86/06296, the structure comprises pyramids with open bases, and lateral surfaces alternately open and closed, connected at their points so as to constitute a multitude of ventilator blades placing the gas in rotation to intensify the contact between the gas and the liquid. A fundamental characteristic of this structure is that it can be made by assembly of perforated and bent metal sheets. This time, the perforation is not only adapted to optimize the circulation of the liquid but also to permit the gas to pass through the bent crossing strips, the rate of perforation being of the order of 50%.
Paradoxically, it is just at this moment at the beginning of serious contestation of the cross corrugated packing that the latter began to be used in the separation of air gases. This relatively late use is explained in part by the high performances of cryogenic plates relative to other plates on the market (HETP, height equivalent to a theoretical plate, of the order of 10 cm, and low pressure drop).
In WO 90/10497, the structure obtained above is improved by causing the surfaces of the pyramids of two successive layers to coincide, which creates transverse channels relative to the strips, and promotes the transverse movement of the mixture. It mentions clearly the interest of a double perforation: one with a checkerboard pattern (hence with 50% of the surface perforated) for the gas, and a secondary perforation in the "closed surfaces" to promote the streaming of the liquid.
This latter patent application gave rise to the Sulzer product "Optiflow".TM. which represents the first embodiment of a new generation, making possible substantially improved performance relative to the now-conventional cross corrugation structures (HETP reduced by an order of 25 to 30% with constant vapor flow rate, or flooding flow rate increased by the order of 25 to 30% with HETP constant).
This patent and these patent applications permit isolating two important directions of research. The first has for its object to improve the flow of the liquid so that the wetted surface will be as large as possible and so that the liquid will distribute itself in all directions whilst remixing in the course of trickling through the packing. The second has for its object to optimize the gas flow, which is to say to obtain a vertical flow as turbulent as possible, without favored flow paths nor regions of low circulation.
Until now, the flow of the liquid phase has been studied in structures of the cross corrugated type. It has been discovered that small diameter perforations (about 10%) promote the passage of the liquid on each side of the strips. Several improvements have been proposed: CA-A-1095827 claims a precise positioning of the holes relative to the bends and WO 94/12258 claims the relative positioning of the strips, by interpenetration of the strips. Thus it appears that the positioning of the holes does not substantially increase the efficiency of the packing because the principal function of the holes is to cause the liquid to pass from one side to the other of the strip. Only the amount of perforation and the diameter of the holes therefore influence efficiency.
The idea of pyramids introduced by WO 86/06296 and WO 90/10497 introduces a new type of perforations: perforations for the passage of gas (representing about 50% of the surface). These perforations permit reducing the pressure drop and creating ventilators favoring the mixing of the gas. These documents are silent as to liquid circulation.
Connecting the pyramids by their points ("summits" and "corners", there is designated by "corners" the points located on the base) has an important drawback: because of there being little material at these points, the mechanical strength of the assembly requires physically connecting these "points" by a mechanical process of the type of clipping, tying, welding or cutting which requires complicated and costly tools, and hence results in a fairly high price. It can also be noted that the number of these connections varies as the number of the pyramids, which is to say as the cube of the inverse of their size, which limits the specific surface economically accessible in this type of packing. On the other hand, the principle of perforating in a checkerboard pattern gives rise to a loss of material of the order of 50%; this is particularly undesirable when the material of the packing is of high cost, a woven material for example.
It also appears that this structure is very aerated and that the HETP could be further reduced if a portion of the waste material was integrated back into the structure without impairing the turbulence of the gas or the rate of wetting of the surface.